Electric Resistance Welded Steel Pipe

ABSTRACT

An electric resistance welded steel pipe excellent in terms of fatigue strength. The steel pipe including a chemical composition containing, by mass %, C: 0.35% or more and 0.55% or less, Si: 0.01% or more and 1.0% or less, Mn: 1.0% or more and 3.0% or less, P: 0.02% or less, S: 0.01% or less, Al: 0.005% or less, N: 0.0050% or less, Cr: 0.1% or more and 0.5% or less, and the balance being Fe and inevitable impurities and a metallic microstructure including pearlite, ferrite, and bainite, in which the area ratio of the pearlite is 85% or more, the total of the area ratios (including 0) of the ferrite and the bainite is 15% or less, and in which a prior austenite grain size is 25 μm or more.

TECHNICAL FIELD

This application is directed to an electric resistance welded steel pipe excellent in terms of fatigue characteristic.

BACKGROUND

In the automotive industry, in order to achieve weight saving and satisfactory stiffness property at the same time, there is a trend toward hollowing driving parts such as a drive shaft which have been manufactured using bar steel to date. As an example of raw materials used for such hollow parts, it is proposed to use a seamless steel pipe, and, for example, Patent Literature 1 discloses a hollow drive axis which is made from a seamless steel pipe as a raw material, having a steel chemical composition controlled to be within a specified range, which is excellent not only in terms of cold workability as indicated by an austenite grain size number of 9 or more after a quenching treatment has been performed but also in terms of hardenability, toughness, and torsion fatigue strength (hereinafter, also simply referred to as fatigue strength), and which realizes a stable fatigue life.

However, in the case of a seamless steel pipe, there is a problem in that, since surface decarburization and surface defects tend to occur due to the method for manufacturing a seamless steel pipe, it is necessary to grind or polish the surface of the pipe in order to achieve satisfactory fatigue resistance, and in that, since a seamless pipe has unevenness and eccentricity in thickness, a seamless pipe is not always suitable for a rotated object.

On the other hand, consideration has been given to using an electric resistance welded steel pipe for a drive shaft, because the problems described above are less likely to occur. For example, Patent Literature 2 discloses a technique for increasing the strength of a steel pipe by using an electric resistance welded steel pipe having a steel chemical composition controlled to be within a specified range as a raw material and by performing a quenching-tempering treatment on a weld of ERW and a portion around the weld as a hardening treatment.

CITATION LIST Patent Literature

-   PTL 1 International Publication No. WO2006/104023 -   PTL 2 Japanese Unexamined Patent Application Publication No.     2002-356742

SUMMARY Technical Problem

However, although an electric resistance welded steel pipe is superior to a seamless steel pipe in terms of dimension accuracy, it is necessary to improve dimension accuracy by performing cold drawing in order to use an electric resistance welded steel pipe for applications such as a drive shaft for which very high dimension accuracy is required. In this case, it is necessary to perform normalizing after cold drawing has been performed. The reasons for that is because it is necessary to solve, for example, the following problems by performing normalizing: (1) a deterioration in toughness due to processing strain in the cold-drawn state, (2) a local increase in hardness in a weld of ERW due to a quenching effect caused by a thermal history in which rapid heating and rapid cooling occur when the welding is performed, and (3) a thin layer called a white layer, in which carbon concentration is low, in the bonded surface of a weld of ERW.

In the case where normalizing is not performed, there is a risk in that, since an electric resistance welded steel pipe has low toughness, brittle failure may occur in a practical use environment. In addition, in the case of a drive shaft, since local stress concentration occurs in a weld of ERW and in the vicinity of the weld due to cyclic shearing stress and bending stress being applied, there is a risk in that fatigue breaking may occur in a short time. Therefore, normalizing is a treatment which is very important in order to use an electric resistance welded steel pipe for a drive shaft and which significantly influences the properties of a steel pipe as a final product.

In the case where high-carbon steel is used as a raw material of an electric resistance welded steel pipe, the metallic microstructure widely varies from ferrite and pearlite to martensite due to a variation in cooling rate after normalizing has been performed. Therefore, since martensite microstructure may be formed, a tempering treatment becomes an indispensable process in order to achieve satisfactory toughness as disclosed in Patent Literature 1 and Patent Literature 2 in the case where high-carbon steel is used as a raw material of an electric resistance welded steel pipe, which results in a problem of an increase in manufacturing cost.

An object of disclosed embodiments is, in order to solve the problems described above, to provide an electric resistance welded steel pipe whose metallic microstructure and tensile strength after normalizing has been performed are less likely to be influenced by a cooling rate when normalizing is performed even in the case where high-carbon steel is used as a raw material of an electric resistance welded steel pipe and with which stable fatigue strength can be achieved.

Solution to Problem

Investigations were diligently conducted in order to solve the problems described above, and as a result, found that, by controlling Al content in steel to be within an appropriate range, the metallic microstructure and tensile strength after normalizing has been performed become less likely to be influenced by a cooling rate after normalizing has been performed and that stable fatigue strength can be achieved. Moreover, it was found that, by controlling the prior austenite grain size to be within an appropriate range, it is possible to increase (1) the strength of pearlite and (2) fatigue crack propagation resistance of ferrite-pearlite steel without changing the tensile strength, which results in an increase in fatigue strength.

Hot-reduced steel pipes (having an outer diameter of 45 mm and a wall thickness of 4.5 mm) were manufactured by using a hot-rolled steel sheets (coiled at a coiling temperature of 650° C.) having a basic chemical composition in accordance with the steel specification SAE1541 (containing 0.42% C,1.5% Mn,0.0035% N) and Al in various amounts as a raw material, by performing roll forming and high-frequency resistance welding on the raw material in order to manufacture electric resistance welded steel pipes (having an outer diameter of 89 mm and a wall thickness of 4.7 mm), and by thereafter performing hot reducing on the formed and welded pipes. Subsequently, by performing cold drawing in order to make cold-drawn pipes (having an outer diameter of 40 mm and a wall thickness of 4.0 mm), and by thereafter performing normalizing (at a temperature of 920° C. for a holding time of 10 minutes and with a cooling rate of 0.5° C./sec. to 3.0° C./sec. after soaking had been performed), product steel pipes were manufactured.

FIG. 1 illustrates the relationship between a cooling rate for normalizing and HV hardness (Vickers hardness). It is clarified that, in the case where the Al content is 0.005% or less, almost constant HV hardness is achieved for a wide cooling rate range, that, in the case where the Al content is 0.007% or more, HV hardness is strongly influenced by the cooling rate, and that, in the case where the cooling rate is low, there is a sharp decrease in HV hardness.

FIG. 2 illustrates the relationship between the Al content and a lamellar spacing, FIG. 3 illustrates the relationship between the Al content and a prior austenite grain size, and FIG. 4 illustrates the relationship between the Al content and torsion fatigue strength. Here, the cooling rate for normalizing was 1° C./sec. The prior austenite grain size increases with decreasing Al content, and the torsion fatigue strength increases along with the prior austenite grain size. It is clarified that, in the case where the Al content is 0.005% or less, such an effect becomes saturated and that the torsion fatigue strength also becomes stable.

FIG. 5 illustrates the results of the cross-section observation of the fracture portion after a fatigue test had been performed, and FIG. 5( a) and FIG. 5 (b) respectively illustrate the fatigue crack propagation situations for a material containing 0.03%-Al and a material containing 0.003%-Al. The crack propagation route is indicated with a white line. It was found that fatigue crack starts from the outer surface side of a pipe and then propagates through a winding path made of soft pro-eutectoid ferrite. In addition, it is presumed that, since the crack meanders in a zig-zag manner, and since the degree of a change in direction increases with increasing apparent pearlite grain size (corresponding to a prior austenite grain), which is surrounded by the pro-eutectoid ferrite, there is an improvement in crack propagation resistance, which results in an increase in fatigue strength.

The reason why the results illustrated in FIG. 1, FIG. 2, and FIG. 3 were obtained is thought to be as follows. That is, since the amount of aluminum nitride, which has been precipitated before normalizing is performed, decreases with decreasing Al content, there is a decrease in the pinning effect of aluminum nitride, which results in a tendency for an austenite grain size to increase in a normalizing process. Since pearlite and ferrite use prior austenite grain boundaries as their transformation sites, in the case where there is a decrease in grain boundary area due to an increase in prior austenite grain size, there is a decrease in the number of transformation sites, which results in a decrease in the fraction of ferrite. In particular, the reason why hardness varies depending on the Al content in a low cooling rate region in FIG. 1 is because, in the case where the Al content is high, since the growth of austenite grains is suppressed in a normalizing process due to the pinning effect of aluminum nitride (AlN) which has been precipitated before normalizing is performed, and, at the same time, since there is an increase in the lamellar spacing of pearlite which is finally formed, there is a decrease in hardness. A decrease in hardness is significant particularly in a low cooling rate region, in which quenching effect is less likely to be realized, and significantly depends on the Al content (the amount of AlN precipitated) in steel. In the case where the Al content is 0.005% or less, since there is a decrease in the amount of aluminum nitride (AlN) precipitated, and since aluminum nitride is dissolved in a normalizing process even if aluminum nitride is precipitated in advance, there is a decrease in pinning effect, which results in a decrease in the lamellar spacing of pearlite due to austenite grains growing easily, and which results in a decrease in a change in hardness depending on a cooling rate.

The relationships of an austenite grain size to a lamellar spacing and strength are thought to be as follows. That is, in the case where an austenite grain size is large, since there is a decrease in the number of pearlite transformation sites (mainly austenite grain boundaries), there is a decrease in pearlite transformation temperature. As a result, it is considered that, since there is an increase in the temperature difference between the pearlite equilibrium transformation temperature and the transformation starting temperature, that is, the degree of undercooling, there is a decrease in lamellar spacing, which results in an increase in the strength of pearlite as expected based on the conventionally-known relationship between a lamellar spacing and the strength of pearlite. As a result, it is considered that, since a fatigue crack becomes less likely to penetrate pearlite microstructure due to an increase in the strength of pearlite, the crack propagates in a zig-zag manner avoiding the pearlite, which results in an increase in fatigue strength due to an increase in fatigue crack propagation resistance.

Disclosed embodiments have been completed on the basis of the knowledge described above and further investigations, and the subject matter of disclosed embodiments is as follows.

[1] An electric resistance welded steel pipe, the steel pipe having a chemical composition containing, by mass %, C: 0.35% or more and 0.55% or less, Si: 0.01% or more and 1.0% or less, Mn: 1.0% or more and 3.0% or less, P: 0.02% or less, S: 0.01% or less, Al: 0.005% or less, N: 0.0050% or less, Cr: 0.1% or more and 0.5% or less, and the balance being Fe and inevitable impurities and a metallic microstructure including pearlite, ferrite, and bainite, in which the area ratio of the pearlite is 85% or more, the total of the area ratios (including 0) of the ferrite and the bainite is 15% or less, and in which a prior austenite grain size is 25 μm or more.

[2] The electric resistance welded steel pipe excellent in terms of fatigue characteristic according to item [1], the steel pipe having the chemical composition further containing, by mass %, one or more selected from among Ti: 0.005% or more and 0.1% or less, B: 0.0003% or more and 0.0050% or less, Mo: 2% or less, W: 2% or less, Nb: 0.1% or less, V: 0.1% or less, Ni: 2% or less, Cu: 2% or less, Ca: 0.02% or less, and REM: 0.02% or less.

Advantageous Effects

According to embodiments, it is possible to obtain an electric resistance welded steel pipe having satisfactory fatigue resistance which is required for a drive shaft.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a relationship between a cooling rate when normalizing is performed and HV hardness.

FIG. 2 is a diagram illustrating a relationship between Al content in steel and lamellar spacing.

FIG. 3 is a diagram illustrating a relationship between Al content in steel and a prior austenite grain size.

FIG. 4 is a diagram illustrating a relationship between Al content in steel and torsion fatigue strength.

FIG. 5A is a diagram illustrating the propagation behavior of a fatigue crack for a material containing 0.03%-Al.

FIG. 5B is a diagram illustrating the propagation behavior of a fatigue crack for a material containing 0.003%-Al.

DETAILED DESCRIPTION

The reasons for the constituent features of disclosed embodiments will be described hereafter.

1. Regarding Chemical Composition

First, the reasons for the limitations on the chemical composition of the steel according to disclosed embodiments will be described. Here, % used when describing a chemical composition always represents mass %.

C: 0.35% or more and 0.55% or less

In the case where the C content is less than 0.35%, it is not possible to achieve satisfactory strength or desired fatigue resistance. On the other hand, in the case where the C content is more than 0.55%, since there is a deterioration in weldability, it is not possible to achieve a stable welding quality of ERW. Therefore, the C content is set to be in a range of 0.35% or more and 0.55% or less, or preferably in a range of 0.40% or more and 0.45% or less.

Si: 0.01% or more and 1.0% or less

There is a case where Si is added for deoxidation, and it is not possible to realize a sufficient deoxidation effect in the case where the Si content is less than 0.01%. At the same time, Si is also a solute strengthening element, and it is necessary that the Si content be 0.01% or more in order to realize such an effect. On the other hand, in the case where the Si content is more than 1.0%, there is a deterioration in the hardenability of a steel pipe. The Si content is set to be in a range of 0.01% or more and 1.0% or less, or preferably 0.1% or more and 0.4% or less.

Mn: 1.0% or more and 3.0% or less

Mn is a chemical element which promotes pearlite transformation and improves hardenability, it is necessary that the Mn content be 1.0% or more in order to realize such effects. On the other hand, in the case where the Mn content is more than 3.0%, there is a deterioration in the welding quality of ERW, and in addition, there is a deterioration in fatigue resistance due to an increase in the amount of residual austenite. The Mn content is set to be in a range of 1.0% or more and 3.0% or less, or preferably in a range of 1.4% or more and 2.0% or less.

P: 0.02% or less

P is an inevitable impurity in embodiments, and the upper limit of the P content is set to be 0.02% or less. There is a tendency for P to be concentrated in a segregation part which is formed when continuous casting is performed and to remain in a hot-rolled steel sheet as a raw material of a pipe. Since the edges of a steel strip are abutted and subjected to upsetting when electric resistance welding is performed, the segregation part in which P is concentrated may be exposed on the outer surface and inner surface of a steel pipe, which results in there being a risk in that cracking occurs when secondary processing such as flattening forming is performed on this part. Therefore, it is preferable that the P content be 0.01% or less.

S: 0.01% or less

S is an inevitable impurity in embodiments, and the upper limit of the S content is set to be 0.01% or less. In the case where the S content is high, there is a deterioration in toughness of raw material, and S combines with Mn in steel to form MnS. Since MnS is elongated in the longitudinal direction of a steel sheet to form a long inclusion in a hot rolling process, there is a deterioration in workability and toughness. Therefore, it is preferable that the S content be 0.005% or less, or more preferably 0.003% or less.

Al: 0.005% or less

Although Al is an important chemical element in embodiments in order to achieve the desired prior austenite grain size accompanied by satisfactory torsion fatigue strength, since, in the case where the Al content is more than 0.005%, a pinning effect is realized in a normalizing process due to an increase in the amount of AlN precipitated, which results in the desired austenite grain size not being achieved due to the growth of austenite grains being suppressed. Therefore, the Al content is set to be 0.005% or less, or preferably 0.003% or less.

N: 0.0050% or less

Since N is a chemical element which contributes to suppressing the growth of austenite grains in a normalizing process as a result of combining with Al to form AlN, it is necessary that the N content be 0.0050% or less in order to suppress such an effect, or preferably 0.0035% or less.

Cr: 0.1% or more and 0.5% or less

Since Cr is a chemical element which decreases the pearlite transformation temperature, there is a decrease in the lamellar spacing of pearlite, which results in an increase in torsion fatigue strength due to an increase in the strength of pearlite. It is necessary that the Cr content be 0.1% or more in order to realize such an effect. On the other hand, in the case where the Cr content is more than 0.5%, since Cr forms oxides, and since the oxides may remain in a weld of ERW, there may be a deterioration in weldability of ERW. Therefore, the Cr content is set to be in a range of 0.1% or more and 0.5% or less, or preferably in a range of 0.15% or more and 0.30% or less.

The basic chemical composition according to embodiments is as described above, and one or more of Ti, B, Mo, W, Nb, V, Ni, Cu, Ca, and REM, which will be described below, may further be added in order to increase strength and fatigue strength.

Ti: 0.005% or more and 0.1% or less

Ti is effective for fixing N in steel in the form of TiN. However, in the case where the Ti content is less than 0.005%, there is insufficient effect of fixing N, and, in the case where the Ti content is more than 0.1%, there is a deterioration in the workability and toughness of steel. In the case where Ti is added, it is preferable that the Ti content be in a range of 0.005% or more and 0.1% or less, or more preferably in a range of 0.01% or more and 0.04% or less.

B: 0.0003% or more and 0.0050% or less

B is a chemical element which improves hardenability. In the case where the B content is less than 0.0003%, there is insufficient effect of increasing hardenability. On the other hand, in the case where the B content is more than 0.0050%, such an effect becomes saturated and there is a deterioration in fatigue resistance due to intergranular fracture being more likely to occur as a result of B being precipitated at the grain boundaries. In the case where B is added, it is preferable that the B content be in a range of 0.0003% or more and 0.0050% or less, or more preferably in a range of 0.0010% or more and 0.0040% or less.

Mo: 2% or less

Since Mo is a chemical element which improves hardenability, Mo is effective for increasing fatigue strength by increasing the strength of steel. It is preferable that the Mo content be 0.001% or more in order to realize such an effect. However, in the case where the Mo content is more than 2%, there is a significant deterioration in workability. In the case where Mo is added, it is preferable that the Mo content be 2% or less, or more preferably in a range of 0.001% or more and 0.5% or less.

W: 2% or less

W is effective for increasing the strength of steel by forming carbides. It is preferable that the W content be 0.001% or more in order to realize such an effect. However, in the case where the W content is more than 2%, since unnecessary carbides are precipitated, there is a deterioration in fatigue resistance and there is a deterioration in workability. In the case where W is added, it is preferable that the W content be 2% or less, or more preferably in a range of 0.001% or more and 0.5% or less.

Nb: 0.1% or less

Nb is a chemical element which improves hardenability and which contributes to an increase in strength by forming carbides. It is preferable that the Nb content be 0.001% or more in order to realize such effects. However, in the case where the Nb content is more than 0.1%, the effects become saturated and there is a deterioration in workability. In the case where Nb is added, it is preferable that the Nb content be 0.1% or less, or more preferably in a range of 0.001% or more and 0.04% or less.

V: 0.1% or less

V is a chemical element which is effective for increasing the strength of steel by forming carbides and which has temper softening resistance. It is preferable that the V content be 0.001% or more in order to realize such effects. However, in the case where the V content is more than 0.1%, the effects become saturated and there is a deterioration in workability. In the case where V is added, it is preferable that the V content be 0.1% or less, or more preferably in a range of 0.001% or more and 0.5% or less

Ni: 2% or less

Since Ni is a chemical element which improves hardenability, Ni is effective for increasing fatigue strength by increasing the strength of steel. It is preferable that the Ni content be 0.001% or more in order to realize such an effect. However, in the case where the Ni content is more than 2%, there is a significant deterioration in workability. In the case where Ni is added, it is preferable that the Ni content be 2% or less, or more preferably in a range of 0.001% or more and 0.5% or less.

Cu: 2% or less

Since Cu is a chemical element which improves hardenability, Cu is effective for increasing fatigue strength by increasing the strength of steel. It is preferable that the Cu content be 0.001% or more in order to realize such an effect. However, in the case where the Cu content is more than 2%, there is a significant deterioration in workability. In the case where Cu is added, it is preferable that the Cu content be 2% or less, or more preferably in a range of 0.001% or more and 0.5% or less.

Ca: 0.02% or less and REM: 0.02% or less

Since Ca and REM are both chemical elements which are effective for suppressing the formation of the origins of cracks which induce a fatigue breaking in a use environment in which cyclic stress is applied by making the shape of non-metal inclusions spherical, these chemical elements may be selectively added as needed. Such an effect is recognized in the case where the content of each of Ca and REM is 0.0020% or more. On the other hand, in the case where the content is more than 0.02%, there is a decrease in cleaning level due to an increase in the amount of inclusions. Therefore, in the case where Ca or REM is added, it is preferable that the content of each of Ca and REM be 0.02% or less. In the case where Ca and REM are added in combination, it is preferable that the total content be 0.03% or less.

The remainder of the chemical composition of the steel according to embodiments other than the constituents described above consists of Fe and inevitable impurities.

2. Regarding Metallic Microstructure

The metallic microstructure according to embodiments is a microstructure in which the area ratio of pearlite is 85% or more and in which the total of the area ratios of ferrite and bainite (including 0) is 15% or less.

In order to increase fatigue strength by increasing fatigue crack propagation resistance as a result of a fatigue crack propagating in a zig-zag manner as described above, it is necessary that the metallic microstructure include mainly pearlite and that the area ratio of pearlite be 85% or more to realize such an effect. On the other hand, in the case where the total of the area ratios (including 0) of soft ferrite and bainite, which is hard but not so effective than pearlite, is more than 15%, there is a decrease in the effect of increasing fatigue strength. Therefore, the area ratio of pearlite is set to be 85% or more, and the total of the area ratios (including 0) of ferrite and bainite is set to be 15% or less.

Prior austenite grain size: 25 μm or more

Since the degree of the deflection of a fatigue crack increases with increasing apparent grain size of pearlite which is surrounded by ferrite layers, there is an improvement in crack propagation resistance. Since ferrite is formed at prior austenite grain boundaries, the apparent pearlite grain size increases with increasing prior austenite grain size. It is necessary that the prior austenite grain size be 25 μm or more in order to improve crack propagation resistance, and there is an insufficient improvement in fatigue crack propagation resistance in the case where the prior austenite grain size is less than 25 μm.

It is conventionally known that the strength of pearlite increases with decreasing lamellar spacing of pearlite. In order to increase the strength of pearlite so that a fatigue crack does not penetrate the pearlite and goes around the pearlite, it is preferable that the lamellar spacing be 170 nm or less, or more preferably 150 nm or less.

Example 1

Hot-reduced steel pipes (having an outer diameter of 45 mm and a wall thickness of 4.5 mm) were manufactured, by performing hot rolling on steel slabs having steel chemical compositions (mass %) given in Table 1 in order to obtain hot-rolled steel strips, by performing roll forming and high-frequency resistance welding on the hot-rolled steel strips in order to manufacture electric resistance welded steel pipes (having an outer diameter of 89 mm and a wall thickness of 4.7 mm), and by thereafter performing hot reducing on the formed and welded pipes. Subsequently, product steel pipes were manufactured, by performing cold drawing in order to obtain cold drawn steel tubes (having an outer diameter of 40 mm and a thickness of 4.0 mm), and thereafter performing normalizing (at a temperature of 920° C. for a duration of 10 minutes and with a cooling rate of 0.5° C./sec. to 3.0° C./sec. after soaking had been performed).

TABLE 1 Steel Chemical Composition (mass %) Grade C Si Mn P S Al N Cr Ti B Mo W Nb V Ni Cu Ca REM Note A 0.42 0.22 1.53 0.009 0.004 0.0030 0.0035 0.20 — — — — — — — — — — Example Steel B 0.42 0.22 1.53 0.009 0.004 0.0030 0.0035 0.20 — — — — — — — — 0.0012 — Example Steel C 0.45 0.22 1.53 0.009 0.004 0.0040 0.0035 0.20 — — — — — — — — — — Example Steel D 0.42 0.22 1.53 0.009 0.004 0.0030 0.0035 0.20 0.015 — — — — — — — — — Example Steel E 0.42 0.22 1.53 0.009 0.004 0.0030 0.0035 0.20 — 0.0015 — — 0.01 — — — — — Example Steel F 0.42 0.22 1.53 0.009 0.004 0.0030 0.0035 0.20 — — 0.1 — — — — — — — Example Steel G 0.42 0.22 1.53 0.009 0.004 0.0045 0.0035 0.20 — — — 0.1 — — — — — — Example Steel H 0.42 0.22 1.53 0.009 0.004 0.0030 0.0035 0.20 — — — — — 0.1 — — — — Example Steel I 0.42 0.22 1.53 0.009 0.004 0.0030 0.0035 0.20 — — — — — — 0.1 0.1 — — Example Steel J 0.42 0.22 1.53 0.009 0.004 0.0030 0.0035 0.20 — — — — — — — — — 0.001 Example Steel K 0.31 0.22 1.53 0.009 0.004 0.0030 0.0035 0.20 — — — — — — — — — — Comparative Example Steel L 0.42 0.22 0.80 0.009 0.004 0.0030 0.0035 0.20 — — — — — — — — — — Comparative Example Steel M 0.42 0.22 1.53 0.009 0.004 0.0070 0.0035 0.20 — — — — — — — — — — Comparative Example Steel N 0.42 0.22 1.53 0.009 0.004 0.0300 0.0035 0.20 — — — — — — — — — — Comparative Example Steel O 0.42 0.22 1.53 0.009 0.004 0.0030 0.0035 0.05 — — — — — — — — — — Comparative Example Steel Annotation: An underlined chemical composition indicates a value out of the range according to disclosed embodiments.

Using a tensile specimen (JIS No. 12 specimen) which had been collected from the product steel pipe so that the longitudinal direction is the axis direction of the steel pipe, tensile strength was determined. In addition, etching was performed so that austenite grain boundaries were exposed in a cross-section in the circumferential direction of the steel pipe in order to determine the austenite grain size. The grain size was determined based on a method of section by taking photographs of 10 microscopic fields using an optical microscope at a magnificent of 400 times, and the average value of the determined values was used as a representative value.

In addition, a lamellar spacing of the pearlite was determined using a method of section, by performing a nital corrosion treatment on a cross-section in the circumferential direction of the steel pipe in the similar way as described above, and by taking photographs of 10 microscopic fields in which cementite layers were arranged as much at a right angle as possible to the paper plane using an electron scanning microscope of 20,000 times power, and the average value of the determined values was used as a representative value.

The fatigue strength σw of the steel pipe was determined by performing a torsion fatigue test under conditions that the frequency was 3 Hz, the wave shape was a sine wave, and the stress ratio R was −1 (reversed vibration). Here, σw was defined as the stress with which a fracture did not occur even after the number of the cycles reaches 2 million. These evaluation results of the properties are given in Table 2 and Table 3.

TABLE 2 Reducing Condition Normalizing Condition Area Fraction of Heating Finish Rolling Reducing Cold Drawing Cooling Microstructure % Pipe Steel Kind of Temperature Temperature Ratio Area Reduction Soaking Rate Pearl- Fer- No. Grade Pipe (° C.) (° C.) (%) (%) Condition (° C./sec.) ite rite Bainite 1 A Reduced 950 830 49 21 920° C. × 0.5 95.0 5.0 0.0 2 Steel 10 minutes 1.0 96.0 4.0 0.0 3 Pipe 2.0 93.0 2.0 5.0 4 3.0 92.0 1.0 7.0 5 B Reduced 950 830 49 21 920° C. × 0.5 94.0 6.0 0.0 6 Steel 10 minutes 1.0 96.0 4.0 0.0 7 Pipe 2.0 92.0 3.0 5.0 8 3.0 90.0 1.0 9.0 9 C Reduced 950 830 49 21 920° C. × 0.5 95.0 5.0 0.0 10 Steel 10 minutes 1.0 95.0 4.0 1.0 11 Pipe 2.0 93.0 2.0 5.0 12 3.0 92.0 1.0 7.0 13 D Reduced 950 830 49 21 920° C. × 0.5 95.0 5.0 0.0 14 Steel 10 minutes 1.0 96.0 4.0 0.0 15 Pipe 2.0 91.0 2.0 7.0 16 3.0 92.0 1.0 7.0 17 E Reduced 950 830 49 21 920° C. × 0.5 95.0 5.0 0.0 18 Steel 10 minutes 1.0 96.0 4.0 0.0 19 Pipe 2.0 92.0 2.0 6.0 20 3.0 90.0 1.0 9.0 21 F Reduced 950 830 49 21 920° C. × 0.5 95.0 5.0 0.0 22 Steel 10 minutes 1.0 96.0 4.0 0.0 23 Pipe 2.0 93.0 2.0 5.0 24 3.0 91.0 1.0 8.0 25 G Reduced 950 830 49 21 920° C. × 0.5 95.0 5.0 0.0 26 Steel 10 minutes 1.0 96.0 4.0 0.0 27 Pipe 2.0 93.0 3.0 4.0 28 3.0 90.0 1.0 9.0 29 H Reduced 950 830 49 21 920° C. × 0.5 95.0 5.0 0.0 30 Steel 10 minutes 1.0 96.0 4.0 0.0 31 Pipe 2.0 93.0 3.0 4.0 32 3.0 91.0 2.0 7.0 Prior Strength TS Austenite Lamellar Tensile Torsion Fatigue Stability Result Pipe Grain Size Spacing Strength TS Strength σw From Cooling No. (nm) (nm) (MPa) (MPa) Rate Note 1 32 164 835 175 ∘ Example 2 31 162 840 175 Example 3 35 161 845 180 Example 4 33 159 850 180 Example 5 34 165 832 175 ∘ Example 6 32 165 833 175 Example 7 35 162 840 175 Example 8 36 158 852 180 Example 9 32 162 840 175 ∘ Example 10 34 164 835 175 Example 11 33 167 825 175 Example 12 31 162 840 175 Example 13 31 164 835 175 ∘ Example 14 29 159 850 180 Example 15 30 164 836 175 Example 16 33 163 838 175 Example 17 35 162 840 175 ∘ Example 18 34 161 845 180 Example 19 35 159 850 180 Example 20 33 163 839 175 Example 21 32 166 830 175 ∘ Example 22 29 162 840 175 Example 23 33 161 845 180 Example 24 36 159 850 180 Example 25 35 162 840 175 ∘ Example 26 36 161 845 180 Example 27 35 163 839 175 Example 28 32 161 843 180 Example 29 33 162 840 175 ∘ Example 30 36 164 835 175 Example 31 35 159 850 180 Example 32 33 157 855 180 Example

TABLE 3 Reducing Condition Normalizing Condition Area Fraction of Heating Finish Rolling Reducing Cold Drawing Cooling Microstructure % Pipe Steel Kind of Temperature Temperature Ratio Area Reduction Soaking Rate Pearl- Fer- No. Grade Pipe (° C.) (° C.) (%) (%) Condition (° C./sec.) ite rite Bainite 33 I Reduced 950 830 49 21 920° C. × 0.5 96.0 4.0 0.0 34 Steel 10 minutes 1.0 94.0 4.0 2.0 35 Pipe 2.0 91.0 3.0 6.0 36 3.0 89.0 2.0 9.0 37 J Reduced 950 830 49 21 920° C. × 0.5 96.0 4.0 0.0 38 Steel 10 minutes 1.0 96.0 3.0 1.0 39 Pipe 2.0 92.0 3.0 5.0 40 3.0 90.0 2.0 8.0 41 K Reduced 950 830 49 21 920° C. × 0.5 75.0 25.0 0.0 42 Steel 10 minutes 1.0 78.0 22.0 0.0 43 Pipe 2.0 80.0 20.0 0.0 44 3.0 82.0 18.0 0.0 45 L Reduced 950 830 49 21 920° C. × 0.5 75.0 25.0 0.0 46 Steel 10 minutes 1.0 76.0 24.0 0.0 47 Pipe 2.0 80.0 20.0 0.0 48 3.0 81.0 19.0 0.0 49 M Reduced 950 830 49 21 920° C. × 0.5 80.0 20.0 0.0 50 Steel 10 minutes 1.0 84.0 16.0 0.0 51 Pipe 2.0 82.0 17.0 1.0 52 3.0 80.0 16.0 4.0 53 N Reduced 950 830 49 21 920° C. × 0.5 79.0 21.0 0.0 54 Steel 10 minutes 1.0 80.0 20.0 0.0 55 Pipe 2.0 81.0 17.0 2.0 56 3.0 81.0 16.0 3.0 57 O Reduced 950 830 49 21 920° C. × 0.5 78.0 22.0 0.0 58 Steel 10 minutes 1.0 78.0 22.0 0.0 59 Pipe 2.0 79.0 14.0 7.0 60 3.0 76.0 16.0 8.0 Prior Austenite Lamellar Tensile Torsion Fatigue Strength TS Stability Pipe Grain Size Spacing Strength TS Strength σw Result From Cooling No. (nm) (nm) (MPa) (MPa) Rate Note 33 32 162 842 175 ∘ Example 34 33 164 836 175 Example 35 34 162 840 175 Example 36 33 159 850 180 Example 37 36 162 840 175 ∘ Example 38 35 161 845 180 Example 39 33 166 830 175 Example 40 36 160 846 180 Example 41 19 196 752 140 x Comparative Example 42 20 185 780 145 Comparative Example 43 21 177 800 145 Comparative Example 44 23 172 812 150 Comparative Example 45 18 191 765 140 x Comparative Example 46 20 185 780 140 Comparative Example 47 22 176 802 150 Comparative Example 48 22 169 820 150 Comparative Example 49 18 197 750 135 x Comparative Example 50 17 187 775 140 Comparative Example 51 20 177 800 150 Comparative Example 52 21 173 810 150 Comparative Example 53 17 200 745 135 x Comparative Example 54 18 189 768 140 Comparative Example 55 19 175 805 150 Comparative Example 56 22 171 815 150 Comparative Example 57 21 202 740 135 x Comparative Example 58 22 185 780 145 Comparative Example 59 21 173 810 150 Comparative Example 60 23 171 815 150 Comparative Example Annotation: An underlined evaluated value indicates a value out of the range according to disclosed embodiments.

Here, regarding strength stability, a case where the deviation (the difference between the maximum value and the minimum value) of tensile strength TS when the cooling rate for normalizing was changed in the range of 0.5° C./sec. to 3.0° C./sec. was 50 MPa or less was judged as satisfactory (◯), and a case where the deviation was more than 50 MPa was judged as unsatisfactory (x).

As Table 2 and Table 3 indicate, it is clarified that the electric resistance welded steel pipes according to embodiments are all excellent in terms of strength stability as indicated by the small deviation of strength caused by the change in the cooling rate for normalizing, had high fatigue crack propagation resistance as indicated by the strength stability, the small lamellar spacing, and the large prior austenite grain size, and stably had high torsion fatigue strength.

On the other hand, in the case of a raw material having a high Al content of more than the range according to embodiments, the tensile strength was low in the case where the cooling rate for normalizing was in the lower range, and the torsion fatigue strength was low. In addition, in the case where the cooling rate was in the higher range, although the difference from the examples according to embodiments in tensile strength was small, the torsion fatigue strength was lower than that of the examples of embodiments. The reason for that is thought to be because of the difference in the prior austenite grain size and because of the difference in the strength of pearlite.

Here, although a hot-rolled steel sheet was used as a raw material of an electric resistance welded steel pipe in the present examples, disclosed embodiments are not limited to the examples, and a cold-rolled steel strip may be used as the raw material of a steel pipe. Also, an ordinary electric resistance welded steel pipe, which has not been subjected to hot reducing, may be used as a steel pipe which is subjected to cold drawing. 

1. An electric resistance welded steel pipe, the steel pipe having a metallic microstructure including pearlite, ferrite, and bainite, and a chemical composition comprising: C: 0.35% or more and 0.55% or less, by mass %; Si: 0.01% or more and 1.0% or less, by mass %; Mn: 1.0% or more and 3.0% or less, by mass %; P: 0.02% or less, S: 0.01% or less, by mass %; Al: 0.005% or less, by mass %; N: 0.0050% or less, by mass %; Cr: 0.1% or more and 0.5% or less, by mass %; and Fe and unavoidable impurities as a balance, wherein an area ratio of the pearlite is in the range of 85% or more, the total of area ratios (including 0) of the ferrite and the bainite is in the range of 15% or less, and a prior austenite grain size is in the range of 25 μm or more.
 2. The electric resistance welded steel pipe according to claim 1, the steel pipe having the chemical composition further comprising at least one of Ti: 0.005% or more and 0.1% or less, by mass %, B: 0.0003% or more and 0.0050% or less, by mass %, Mo: 2% or less, by mass %, W: 2% or less, by mass %, Nb: 0.1% or less, by mass %, V: 0.1% or less, by mass %, Ni: 2% or less, by mass %, Cu: 2% or less, by mass %, Ca: 0.02% or less, by mass %, and REM: 0.02% or less, by mass %.
 3. The electric resistance welded steel pipe according to claim 1, wherein the tensile strength of the steel pipe is in the range of 825 MPa to 855 MPa.
 4. The electric resistance welded steel pipe according to claim 1, wherein the torsion fatigue strength of the steel pipe is in the range of 175 MPa to 180 MPa.
 5. The electric resistance welded steel pipe according to claim 1, wherein the prior austenite grain size is in the range of 29 nm to 36 nm. 